1. Field of the Invention
The present invention relates to a noise reduction apparatus for preferably removing noise components contained in a video signal without deteriorating the original video signal.
2. Description of the Related Art
As noise reduction apparatuses, there are cyclic type noise reducers, noncyclic type noise reducers, and noise reducers employing coring circuits. These cyclic and noncyclic type noise reduces utilize such characteristics that a video signal owns high self-correlation along a temporal direction, whereas a noise component owns substantially no self-correlation. The last-mentioned noise reducers clip a high frequency very small component to improve S/N.
The typical noncyclic type S/N improving apparatus and the typical cyclic type S/N improving apparatus are described in, for instance, Japanese publication "Digital Signal Processing for Image" written by Fukinuki and published by NIKKAN KOGYO Newspaper Publisher, pages 115 to 118. FIG. 18 is a schematic block diagram showing the arrangement of this conventional IIR type noise reduction apparatus. Also, FIG. 19 represents one typical characteristic example of the nonlinear limiter 503 shown in FIG. 18.
In the conventional cyclic type noise reduction apparatus with the above-described arrangement, when the frame difference signal is small, the nonlinear limiter 503 returns this frame difference signal to the original signal while this frame difference signal is regarded as noise. When the frame difference signal is large, the nonlinear limiter 503 performs no process operation while this frame difference signal is regarded as no movement. As a consequence, the S/N ratio of the still image region is improved. The larger the value of symbol "a" shown in FIG. 19 becomes, the wider the range to be processed by the noise reduction becomes. Therefore, the S/N improving amount is increased. However, since the movement component is also smoothed, the leaving picture trail will occur.
FIG. 20 is another block diagram showing the arrangement of the conventional noise reduction apparatus with employment of the coring circuit. The noise reduction apparatus with using the coring circuit shown in FIG. 20 is one example of such a noise reducer without the processing operation along the temporal (time base) direction within the same field. One example of the nonlinear limiter 515 indicated in FIG. 20 is represented in FIG. 19.
In the conventional noise reduction apparatus with using the coring circuit having the above-described arrangement, when the high frequency range component of the input picture (image) signal is small, this picture signal is regarded as noise and thus is returned to the original signal by the nonlinear limiter 515. To the contrary, when the high frequency range component of the input picture signal is large, this picture signal is regarded as the original signal component, and therefore no process operation is carried out by the nonlinear limiter 515. As a result, the S/N ratio is improved. The larger the value of symbol "a" in FIG. 19 becomes, the wider the range to be processed by the noise reduction becomes. Accordingly, the S/N improving amount is increased. However, a large blur of the picture will occur in the high frequency range component of the original video (picture) signal.
As described above, however, in the arrangement of the noise reduction apparatus utilizing the correlation along the temporal (time base) direction, since the decision as to whether the input video signal corresponds to the movement signal or noise is made based upon the level of the inter frame difference signal (i.e., a difference between one frame signal and the subsequent frame signal), such an input video signal containing the movement components having the small amplitudes is judged as noise. Thus, the blur would occur in the picture with very small movement. Also, in the above-described conventional noise reduction apparatus with employment of the coring circuit, the judgement between the high frequency range component and the noise is established based upon the level of the high frequency range component. As a consequence, with respect to such a video signal containing a high frequency range component having a small amplitude, this high frequency range component is judged as noise. Thus, the blur would occur in a very small signal component of the original video signal.
Also, in the video system employing the nonlinear calculating means for suppressing the low level signal of the video signal, it is rather difficult to separate the video signal from the noise in the low-level region, as compared with the high level region. In particular, in a natural image such as a scene, there are large possibilities that fine signal components are contained in a region where a brightness level is low and a color level is low. Under such a circumstance, there is a problem of how to minimize the blur so as to improve the S/N ratio.
That is, in accordance with the arrangement of the conventional noise reduction apparatus, since the discrimination between the signal and the noise in the low level signal range is not sufficiently performed, the blur would occur in the video signal. To suppress these blurs, the value of symbol "a" in the characteristic of the nonlinear shown in FIG. 19 is made small, and the S/N improving amount should be decreased. Therefore, there is another problem that the S/N improving effects in the still image region and the flat image portion are lowered.